1. Field of the Invention
Embodiments disclosed herein relate generally to matrix body drill bits and the methods for the manufacture of such drill bits. In particular, embodiments disclosed herein relate generally to use of multiple matrix materials in a bit.
2. Background Art
Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation. In PDC bits, polycrystalline diamond compact (PDC) cutters are received within the bit body pockets and are typically bonded to the bit body by brazing to the inner surfaces of the pockets. Bit bodies are typically made either from steel or from a tungsten carbide matrix bonded to a separately formed reinforcing core made of steel.
Matrix bit bodies are typically formed of a single, relatively homogenous composition throughout the bit body. The single composition may constitute either a single matrix material such as tungsten carbide or a mixture of matrix materials such as different forms of tungsten carbide. The matrix material or mixture thereof, is commonly bonded into solid form by fusing a metallic binder material and the matrix material or mixture.
The drill bit formation process typically includes placing a matrix powder in a mold. The mold is commonly formed of graphite and may be machined into various suitable shapes. Displacements are typically added to the mold to define the pockets. The matrix powder may be a powder of a single matrix material such as tungsten carbide, or it may be a mixture of more than one matrix material such as different forms of tungsten carbide. The matrix powder may include further components such as metal additives. Metallic binder material is then typically placed over the matrix powder. The components within the mold are then heated in a furnace to the flow or infiltration temperature of the binder material at which the melted binder material infiltrates the tungsten carbide or other matrix material. The infiltration process that occurs during sintering (heating) bonds the grains of matrix material to each other and to the other components to form a solid bit body that is relatively homogenous throughout. The sintering process also causes the matrix material to bond to other structures that it contacts, such as a metallic blank which may be suspended within the mold to produce the aforementioned reinforcing member. After formation of the bit body, a protruding section of the metallic blank may be welded to a second component called an upper section. The upper section typically has a tapered portion that is threaded onto a drilling string. The bit body typically includes blades which support the PDC cutters which, in turn, perform the cutting operation. The PDC cutters are bonded to the body in pockets in the blades, which are cavities formed in the bit for receiving the cutting elements.
The matrix material or materials determine the mechanical properties of the bit body (in addition to being partly affected by the binder material used). These mechanical properties include, but are not limited to, transverse rupture strength (TRS), toughness (resistance to impact-type fracture), hardness, wear resistance (including resistance to erosion from rapidly flowing drilling fluid and abrasion from rock formations), steel bond strength between the matrix material and steel reinforcing elements, such as a steel blank, and strength of the bond to the cutting elements, i.e., braze strength, between the finished body material and the PDC cutter. Abrasion resistance represents another such mechanical property.
According to conventional drill bit manufacturing, a single matrix powder is selected in conjunction with the binder material, to provide desired mechanical properties to the bit body. The single matrix powder is packed throughout the mold to form a bit body having the same mechanical properties throughout. It would, however, be desirable to optimize the overall structure of the drill bit body by providing different mechanical properties to different portions of the drill bit body, in essence tailoring the bit body. For example, wear resistance is especially desirable at regions around the cutting elements and throughout the outer surface of the bit body while high strength and toughness are especially desirable at the bit blades and throughout the bulk of the bit body. However, unfortunately, changing a matrix material to increase wear resistance usually results in a loss in toughness, or vice-versa.
Further, in packing the matrix powder materials into the mold, the geometry of the bit (and thus mold) make it difficult to place different matrix materials in different regions of a bit because there is little or no control over powder locations in the mold during assembly, particularly around curved surfaces. According to the conventional art, the choice of the single matrix powder represents a compromise, as it must be chosen to produce one of the properties that are desirable in one region, generally at the expense of another property or properties that may be desirable in another region.
Accordingly, there exists a continuing need for developments in matrix bit bodies to improve wear resistance and toughness in the regions of the bit in which these properties are desirable.